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I helped pour concrete for a living, so let me advise you, its not how thick you make it as much as the amount of steel that counts, a perfect example, the church i attend built a 30 X 60 ft. picnic shelter and wouldn't take any advice , poured it 1 foot thick, no steel, sealed every 2 years, now 10 years later its cracked in every direction and wide cracks at that.